The Syrian Electronic Army is believed to be behind the Associated Press Twitter account hack which said President Barack Obama had been injured in an explosion at the White House.
The tweet, which briefly sent share prices on the Dow Jones average down from 14,703 to 14,554, was sent via the news agency's official account which has nearly two million followers.
AP later suspended the account and said the fake tweet was sent after "hackers made repeated attempts to steal the passwords of AP journalists".
After the message was retweeted thousands of times, White House press secretary Jay Carney was forced to confirm to reporters that "the president is fine" and no explosions occurred at the White House.
The FBI has confirmed they are now launching an investigation into the bogus tweet.
The Syrian Electronic Army has claimed responsibility for hijacking the account. The group wrote on its official website: "This small tweet created some chaos in the United States in addition to a decline in some U.S. stocks".
The group describes itself as a supporter of Syrain President Basahr al-Assad. Its members are thought to be responsible for a series of attacks on news websites and twitter accounts. Earlier this week, the group simultaneously took over the accounts of Fifa president Sebb Platter and the official FIFA World Cup account.
In a series of posts, the accounts claimed that Blatter was due to step down from his role due to corruption charges against him. The tweets claim that Blatter accepted bribes from a Qatari prince so the country could host the 2022 World Cup.
The group also claimed responsibility for the recent hijacking of numerous Twitter accounts associated to US news network CBS, including the 60Minutes account, which tweeted: "The American people have been lied to by their media about Syria" and "Obama wants to destroy the Syrian and American people. We must stop this beast."
The hackers also took over the account of the Agence France Press news agency's photo department, posting a series of photos which allegedly depicted people showing their support for Assad.
The BBC's official weather account was targeted with a series of bizarre tweets including "Saudi weather station down due to head on collision with camel" and "chaotic weather forecast for Lebanon as the government decides to distance itself from the Milky Way".
The BBC Radio Ulster and BBC Arabic accounts were also compromised by the Syrian Electronic Army.
The BBC said the accounts were hacked after a series of "phising emails" were sent to its employees. A similar technique appears to have been used to get into the AP Twitter account.
Following the White House explosion tweet, Mike Baker, a reporter for the news organisation, tweeted it came "less than an hour after some of us received an impressively disguised phishing email".
Journalists and media commentators have pointed out how a change in the social network's security - such as two-step verification - could prevent such hacks from happening again.
Other recent attacks include the Al Jazeera and National Public Radio websites. The Syrain Electronic Army does have its own Twitter account, after repeated suspensions. The current account is thought to be the sixth incarnation.
The pro-Assad group describes itself on its website as: "A group of enthusiastic Syrian youths who could not stay passive towards the massive distortion of facts about the recent uprising in Syria, and this distortion is carried out by many Facebook pages that deliberately work to spread hatred and sectarian intolerance between the peoples of Syria to fuel the uprising."